In precision machine motion, the deviation from the ideal motion path is called runout. For linear motions, the ideal path is typically a straight line. Displacements and angular pitching of the moving stage from this straight path represent runout from the ideal path. In rotational applications, the typical ideal path is a perfect circle relative to a defined axis. In this case, eccentricity of the circular motion, angular wobble of the axis, and distortions of the circular shape of the motion represent runout from the ideal path. Examples of such devices are air bearing spindles, hard disk drive bearing spindles, motors, and shafts.
Measuring runout is an important step towards improving precision machine motion. For example, the tighter track spacings demanded by the increase in the capacity of hard disk drives requires less runout in the rotation bearings in the disk drives. Less runout also allows disk drives to operate with higher throughputs.
Runout has two commonly measured components. First, (in rotating applications) repeatable runout (RRO) is the deterministic motion that occurs on every revolution of a rotating body, (or each pass of a translating body in linear applications). Repeatable runout can be caused, for example, by a rotating disk that is not absolutely round, or which is mounted slightly off-center, or by the nicks and chips at the edge of the rotating body. Non-repeatable runout (NRRO), on the other hand, is the stochastic component of the motion, and can be caused by such things as excessive drive vibration, play in the bearings, imperfectly shaped bearing balls, turbulence in fluid bearing films, or contamination in or on the bearing surfaces,
Runout is measured with sensitive instruments that detect the position of a moving object, such as the edge of a rotating disk. One sensitive instrument commonly used to measure runout is the capacitance probe, which is also referred to as a capacitance gauge. The small overall size of the capacitance probe relative to other measurement candidates allows it to be positioned in confined spaces, such as inside the housing of small disk drives or in a track on a linear translation stage.
Using a rotating disk as an example, a sequence of measurements is made on a time-lapsed basis at a number of points on the surface as it rotates. The rotational position of the object can be determined by triggering a clock with an external encoder, or from an electrical signal provided by the motor driver (motor commutation).
The choice of size of the capacitance probe tip represents a trade-off between resolution in the direction of motion and sensitivity of the measurement itself. The finite probe tip size and limited bandwidth of capacitance probes limits one's ability to adequately resolve the spatial topography of the surface at the rotational speeds at which modern bearings are operated. The electrical response time of the capacitance probe limits the meaningful maximum acquisition rates to between ten thousand and one hundred thousand samples per second. Thus, a capacitance probe will always suffer from a limited lateral resolution. For slow rotation speeds or slow linear stage feeds the lateral resolution is hampered by the finite probe tip size, and in the case of fast rotation speeds the lateral resolution suffers from the limited capacitance probe bandwidth.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.